This invention relates to improved catalysts for the dehydrogenation of hydrocarbons to corresponding more-unsaturated hydrocarbons, more particularly, to the production of vinyl aromatic hydrocarbons from alkyl aromatic hydrocarbons and to the production of olefins from the corresponding more saturated aliphatic hydrocarbons.
The vinyl benzenes and butadienes play a particularly important role in the preparation of synthetic rubbers, plastics and resins. The polymerization of styrene, for example, with various comonomers such as butadiene to produce synthetic rubbers is well known as is the polymerization of styrene to produce polystyrene resins.
Styrene and butadiene are typically produced from ethylbenzene and butylene, respectively, by dehydrogenation over solid catalysts in the presence of steam and at temperatures ranging from 500.degree. C. to 800.degree. C. The class of catalysts found to be the most effective for this process contains a predominant amount of iron oxide, promoted by potassium carbonate, and stabilized by chromium oxide. Any improvement which results in either increasing the selectivity (moles of desired product produced per mole of reactant reacted) or the conversion (moles of reactant reacted per mole of starting material) without lowering the other is economically attractive since the result is that the yield (moles of desired product produced per mole of reactant) of the product has been increased. An increase of only a few tenths of a percent in the selectivity can result in a substantial savings of starting materials while an increase in conversion can substantially reduce capital expenditure and energy consumption.
The prior art catalysts used in the dehydrogenation of alkyl aromatics to alkenyl aromatics, for example, ethylbenzene to styrene as previously discussed, are widely known. Early dehydrogenation patents describe the use of cerium oxide as the major active ingredient. Thus, for example, U.S. Pat. No. 1,985,844 discloses the use of cerium oxide precipitated on broken pieces of clay to dehydrogenate ethylbenzene below atmospheric pressure. U.S. Pat. No. 2,036,410 discloses the use of cerium oxide promoted with oxides of tungsten, uranium, and molybdenum.
Various catalysts, the dehydrogenation conditions and other operating data are disclosed by Pitzer in U.S. Pat. No. 2,866,790 relating to the use of catalyst composition including potassium carbonate, chromium oxide, and iron oxide. Other catalysts and procedures are also shown by Gutzeit, U.S. Pat. Nos. 2,408,140; Eggertsen, et al, 2,414,585; Hills, et al, 3,360,579; 3,364,277: and 4,098,723.
Belgium Pat. No. 811,145 discloses the use of cerium in promoted iron oxide dehydrogenation catalysts. Belgium Pat. No. 811,552, based on the principal patent above, discloses the use of cerium and molybdenum in potassium promoted iron oxide dehydrogenation catalysts. In these two patents iron oxide is 50-80% by weight, preferably 55-65% by weight. Cerium oxide is 0.5-10% by weight, most preferably 4-6% by weight. Potassium carbonate is present in a concentration of from 1-40% by weight.
O'Hara, in U.S. Pat. No. 3,904,552, discloses the use of cerium and molybdenum in alkali promoted iron oxide dehydrogenation catalysts. The iron oxide is employed in the catalyst at 30-90% by weight, more preferably 55-90% by weight. O'Hara's preferred alkali metal compound is potassium carbonate and is employed in amounts in the range of 1.0-40%, preferably 5-25% by weight. Cerium oxide is employed in the range of from 0.5-10% by weight, most preferably 4-6% by weight.
The use of cerium oxide in alkali promoted dehydrogenation catalysts with iron oxide percentages as low as 20% by weight has been disclosed. Thus, MacFarlane, in U.S. Pat. No. 3,223,743, teaches that it is very difficult to obtain a catalyst which has both high selectivity and high activity since, as one increases, the other usually decreases. To overcome this he teaches the use of two layers of catalyst, the first layer with high selectivity (lower activity) and a second layer with high activity (lower selectivity). The first layer with high selectivity and lower activity may contain 20-80% by weight iron and 0.5-5% by weight cerium oxide.
Riesser, in U.S. Pat. No. 4,144,197, discloses the use of vanadium to improve the selectivity of a potassium promoted dehydrogenation catalyst containing 20-95% by weight ferric oxide and 0.01-50% by weight cerium oxide. Data in this patent is consistent with the teaching of MacFarlane with respect to vanadium addition since, by increasing the amount of vanadium, the selectivity of the catalyst is increased at the sacrifice of activity. Another patent disclosing the use of oerium as a promoter in iron oxide based catalysts is U.S. Pat. No. 4,467,046.
The advantage of having a promoted dehydrogenation catalyst containing both cerium and potassium in high concentrations together with low concentrations of iron as related to the known art has not previously been recognized. Thus, catalysts containing more than 10% by weight cerium as cerium oxide and more than 40% by weight potassium as potassium carbonate and less than 30% iron as iron oxide have not been disclosed.
It has now been discovered that a highly active and highly selective dehydrogenation catalyst which contains high concentrations of both cerium and potassium compounds and small amounts of iron, but without vanadium, can be achieved using only a single catalyst layer. The catalytic components are optionally promoted with molybdenum and/or chromium compounds. From these components unitary structures are made, i.e. pellets or extrudates, by adding hydraulic cement and other binders. The catalyst pellets are then calcined before use in a dehydrogenation process.